1. Field of the Invention
This invention relates to a system for automatically positioning a workpiece in a machining operation at a succession of coordinate points in an X-Y coordinate plane and, more particularly, to a system for automatically positioning a plurality of printed circuit boards and drilling holes therein under the control of an automatic control unit.
2. Description of the Prior Art
Printed circuit boards are widely used in the electronics field and in related technologies. Such printed circuit boards are typically laminated of at least a substrate layer and one or more layers of copper. The boards are customarily drilled with holes for the insertion of various electrical components to be mounted on the boards. The holes must be drilled, not punched, in a controlled fashion to limit damage to the areas surrounding the holes and to avoid smearing between layers. Each circuit board with typically have a relatively large number of holes to be drilled through it.
It will therefore be recognized that the drilling of holes in printed circuits is a tedious and time-consuming task. Various systems have been devised in order to automate the procedure. A plurality of boards may be mounted in a single work station for drilling simultaneously. Numerically controlled drill-positioning systems are in present use commercially, employing a tape-driven control unit in conjunction with a movable drill head which repeats the drilling operation at the various positions to which it is directed by the control unit. The tape fed into the control unit contains the program indicating the number and position of holes to be drilled in the circuit board. However, such a system is necessarily restrictive in the number of circuit boards which can be processed in a given time period since for quality production only a few boards can be stacked together for simultaneous drilling. The use of a control unit for each station increases the cost of the drilling system, and the mechanism for positioning the drill is not only expensive to provide on a per-unit basis but is complex and prone to breakdown.
Other types of automatic positioning systems for the automatic drilling of holes in printed circuit boards which eliminate the need for control by a tape driven control unit are known. An example of such a system which uses a master template and a photocell, hole-location system is disclosed in the Marantette et al U.S. Pat. No. 3,532,893. However, while eliminating the need for the expensive and complex tape-driven control unit of the first type of system described hereinabove, systems of this type depend upon a plurality of servo devices for positioning the boards at the respective work positions, thus necessarily introducing a factor of cost from the replication of such items at multiple work positions. Moreover, such systems are subject to their own position error problems, particularly the photo detection systems which rely on a null signal for determining the selected position for drilling.
A system is desired which is responsive to an automatic control unit driven by a paper tape or the like so as to develop the precision in position location which is possible from the use of stepping motors but without the associated cost which is necessarily involved where such a control unit is required for each hole drilling machine. If a plurality of hole drilling machines could be controlled from a single control unit with the precision which is required in such an operation, the cost per work position could be reduced by spreading the control unit cast over the number of work positions employed, thus reducing the cost of drilling the holes, and the rate of production of drilling circuit boards could be increased, since more circuit boards would be drilled at the same time. I have devised such a system.